Accurate measurements of pressure, stress, force, or deflection are required in many technical applications. Piezoactive materials generate a change in resistance or produce a voltage upon application of pressure or deformation and are a convenient choice to enable such measurements. These materials may also be used as actuators or force generators which exhibit deflection or vibration as a voltage is applied. Similarly, changes in capacitance can also be used to measure pressure, stress, force or deflection. Most capacitors are designed to maintain a fixed physical structure. However, various factors, such as pressure, can change the structure of the capacitor, and the resulting change in capacitance can be used to sense those factors.
In many cases it is desirable to introduce such functionality directly to the surface of an object which is frequently three-dimensional, and/or non-planar. Screen printing can be utilized to apply a pressure sensitive material in a specific area or pattern of a three dimensional object or substrate, but only if the object is sufficiently stiff or has a stiff backing, and also has a regular geometry which is substantially planar or cylindrical. If the object is flexible, which is the case with most polymeric medical devices, a stiff backing material must be held behind the object to enable accurate and usable screen printing. If the object is non-planar or irregular in shape, such a backing would be extremely inconvenient and could be impossible to apply.
It is possible to envision producing such sensors or actuators and then applying or gluing them directly to the three dimensional surface of the object; however, this carries the risk that the sensors may become dislodged during use and, in the case of medical devices, that sharp edges at the glued interface may cause discomfort or trauma as the device is inserted or used.
U.S. Pat. No. 5,785,051 to Lipscher et al. describes an endotracheal tube with a piezoelectric ultrasonic transducer applied on the outside or the inside of the cuff. In this case the piezoelectric element is manufactured separately and affixed to the device. This approach poses difficulties in identification and use of effective adhesives. If the transducer is affixed inside the cuff, it is very difficult to access and control accurate placement; while, if it positioned on the outside of the cuff, the aforementioned sharp edges can cause tissue damage during intubation.
U.S. Pat. No. 5,046,503 to Schneiderman describes a similar transducer bonded to a catheter to aid in measurement of blood flow. In this case, the concept of “painting on” the connecting leads is disclosed, but the transducer still must be affixed in a separate step leading to the above-noted difficulties.
U.S. Pat. No. 5,611,807 to O'Boyle describes a method in which an entire medical balloon is comprised of a piezoelectric polymer. The polymer is directly extruded into a balloon shape, and the entire balloon is used as an ultrasonic vibration source to treat stenosis. One major disadvantage of such an approach is that piezoactive polymers are unable to withstand the inflation pressures that can be applied to balloons of more commonly used materials such as polyethylene (terephthalate) and polyurethane. Another disadvantage of this approach is that the most effective piezoactive materials are also very expensive. As a result, making an entire medical device from one is not cost effective since only a small fraction of the area is generally required to be piezoactive.
WO 1998/050773 to Charych et al. describes a cantilever biosensor which responds to a chemical stimulus or binding event with an electrical output. The cantilever beam is comprised of a piezoelectric material, and the entire sensor is manufactured using a subtractive MEMS approach. Such an approach does not lend itself easily to polymeric substrates with complex shape, but rather is designed for flat substrates. In addition, the starting materials and processes are difficult and complex compared with direct write techniques using common ink or paste materials.
U.S. Patent Application Publication No. 2008/0009750 to Aeby et al. describes a unique force sensing approach used for detection of pressure on catheters. Reflectors, optical sources, and detectors are mounted triaxially in a housing which is mounted on the tip of a catheter. Deformation due to contact force of the catheter tip results in differences in light reflected from the surfaces which can then be detected through a plurality of fiber optics. While this approach is creative, it is cumbersome, expensive to execute, and limited in the types of surfaces to which it could be successfully affixed.
The present invention is directed to overcoming these and other deficiencies in the art.